(a) Field
Embodiments of the present invention relate to a display device and a driving method thereof.
(b) Description of the Related Art
An organic light emitting diode display, which is a type of flat panel display apparatus, displays an image by using an organic light emitting diode which generates light by recombining electrons and holes. The organic light emitting diode display has a rapid response speed, low power consumption, and excellent emission efficiency, luminance, and viewing angles, and thus has received much attention.
Generally, organic light emitting diode displays are classified into passive matrix organic light emitting diode displays (PMOLEDs) and active matrix organic light emitting diode displays (AMOLEDs) according to a driving mode (or driving method) of the organic light emitting display.
The passive matrix type OLED display (PMOELD) uses a driving mode in which a positive electrode and a negative electrode are formed to cross each other, with an organic light emitting diode coupled between the positive electrode and the negative electrode and wherein the organic light emitting diode emits light when the negative electrode and the positive electrode line are concurrently driven, and the active matrix type OLED display (AMOELD) uses a driving mode in which a thin film transistor and a capacitor are integrated in each pixel to maintain (or store) a voltage in the capacitor. The passive matrix type OLED display has a simple structure and is relatively inexpensive, but it is difficult to implement a large-sized or high-precision panel. On the other hand, the active matrix type OLED display may implement the large-sized or high-precision panel, but controlling AMOLED displays is technically difficult and the costs are relatively high.
In view of its resolution, contrast, and operation speed, the active matrix type organic light emitting diode display (AMOLED) in which light is selectively emitted for each unit pixel has become mainstream.
In one pixel of the active matrix OLED (hereinafter, referred to as an organic light emitting diode display), the emission degree (or brightness or gray level) of the organic light emitting diode is controlled by controlling a driving transistor which supplies a driving current to the organic light emitting diode according to data voltage.
In a display panel of the organic light emitting diode display, differences in the threshold voltages and current mobilities among a plurality of driving transistors may occur. The differences may occur due to characteristics of the materials (e.g., the polysilicon), and manufacturing process, method, and environment of the driving transistors. In addition, the differences may occur due to deterioration of the driving transistor as the organic light emitting diode display is used over time.
Although the same data voltage is applied to each pixel circuit, the output emission degrees (e.g., brightnesses or gray levels) of the pixels vary due to non-uniformities in the threshold voltage characteristics of the driving transistors. Accordingly, a spot phenomenon (that may appear as, for example, relatively dark, sand-like particles) occurs on a bright screen. For example, when the threshold voltages of the driving transistors are not uniform, although the same data voltage is applied, the gate-source voltage Vgs output of the driving transistors, which is directly associated with a driving current supplied to the organic light emitting diode, varies. Accordingly, an accurate gray level is not expressed according to a data signal and a spot occurs, and, as a result, the quality of the display is deteriorated.
Technology for correcting for image degradation through the compensation of threshold voltage variation of the driving transistors has been developed, but recently, as display panels have grown in size and thus the display panel is driven at a higher speed, there are problems in that a compensation time is limited (e.g., the time available for compensation is short) and gray spots of the display panel may proliferate.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.